When cement slurry is to be pumped down a casing string and into the annulus outside the casing, it is a common practice to incorporate a float collar in the string a selected distance above the bottom. The collar has an upwardly closing ball check valve which allows cement to flow downward, but which prevents upward flow due to "U-tubing". The device is called a float collar because as the casing string is being run into the well bore, the check valve keeps the casing empty of fluids to provide a buoyancy force which reduces the hook load that must be suspended at the rig. The standard float collar has several disadvantages, one being that the check valve provides a considerable restriction to downward flow of cement slurry during a cementing operation. Another disadvantage is that the ball check valve arrangement frequently fails to prevent back-flow of cement because of trash and debris that gets lodged between the ball and its seat.
An object of the present invention is to provide a new and improved stop collar apparatus that cooperates with a cement displacement plug to positively stop cement flow in either direction when the plug reaches a certain distance above the bottom of the casing.
Another object of the present invention is to provide a new and improved stop collar of the type described that is full-bore with respect to cement displacement, and which cooperates with the drive plate of a displacement plug that lands in the collar to seal off the casing bore against flow in either direction.